Amblyopia is the most frequent cause of visual loss in childhood and thus is a significant developmental and clinical disorder. Adult amblyopes show marked deficits in position discrimination, which differ between strabismic and anisometropic amblyopes. A conceptual and quantitative framework based upon spatial filtering and spatial sampling has been developed for modelling position discrimination in normal vision. This proposal applies the framework to amblyopic observers. The first goal is to elucidate the mechanisms underlying the abnormal position sense of the amblyopic eye. We propose to make quantitative psychophysical measurements of the components of our model, the "primitives" (contrast response and spatial sampling grain of the putative mechanisms) for position discrimination in normal and amblyopic vision - and to analyze the results in terms of the model. Our hypothesis is that reduced position sensitivity of anisometropic amblyopes is accounted for by their reduced contrast response function and that the additional loss of position acuity shown by strabismic amblyopes is a consequence of spatial undersampling. The second goal is to test the hypothesis that the normal peripheral visual field is an analog for strabismic amblyopia. Our research shows that the normal periphery, like the central field of strabismic amblyopes, has markedly reduced position acuity. Compairsons between normal peripheral versus amblyopic performance will provide insights into the mechanisms underlying amblyopia. We propose to test the hypothesis that both normal peripheral and strabismic amblyopic central visual systems exhibit spatial undersampling. In order to determine how these visual systems may differ, we also propose to compare their spatio-temporal discrimination functions, whcih our preliminary studies suggest may be quite different. A third goal is to test the hypothesis that anisometripic amblyopes have residual stereopsis when the effective contrast in the two eyes is equal, and to examine the effects of degradation upon this stereoscopic position discrimination. The fourth goal is to extend our psychophysical experiments to children with amblyopia. This will enable us to I) increase our data base, II) determining how the laboratory results of adult amblyopes relate to those of young children, III) examine developmental trends to determine whether the sampling losses precede losses in contrast sensitivity and spatial resolution and IV) monitor the effects of treatment on spatial discrimination.